Preheat oven to 350°F. Line two baking sheets with parchment paper.
Cream softened butter, brown sugar, and granulated sugar until light and fluffy, 2–3 minutes.
Beat in eggs one at a time, then mix in vanilla until smooth.
Whisk flour, cocoa, baking soda, salt, and espresso powder in a separate bowl.
Add dry ingredients to wet and mix on low just until combined. Do not overmix.
Fold in mini chocolate chips. Let dough rest 5 minutes to thicken.
Scoop 1½-tablespoon mounds, spacing 2 inches apart on the sheets.
Bake 9–11 minutes, until edges are set and centers look slightly soft.
Cool on the sheet 5 minutes, then transfer cookies to a rack to cool completely.
Warm cream in a small saucepan until steaming. Remove from heat; add chocolate chips and butter.
Let stand 2 minutes, then whisk until glossy and smooth to make ganache.
Spoon or spread ganache over cooled cookies and top with rainbow chips. Let set before serving.
Notes
For extra fudgy centers, pull the cookies when the middles still look slightly underbaked. Chill cooked, undecorated cookies up to 3 days or freeze up to 2 months; glaze after thawing. Try peanut butter chips in the ganache for a fun twist.This recipe is an original creation inspired by classic Cosmic Brownie Cookies Recipe flavors. All ingredient ratios and instructions are independently developed.